This invention relates generally to fuel rods employed in nuclear reactors. More particularly, the present invention relates to fuel rods having a zirconium-alloy cladding tube which contains fuel pellets.
Fuel rods having outer cladding tubes are mounted in support grids of the reactor fuel assembly. Because of the harsh environment of the fuel assembly where the surrounding water temperature is typically 400.degree. C. and the water has a relatively high pressure, the cladding tube is susceptible to wear and corrosion. At the lower portions of the reactor assembly, the cladding tubes are exposed to debris fretting. In addition, there are severe wear forces at the location of the grid support.
A number of advancements have been introduced in some industrial applications to improve the ability of metallurgical thin film to combat wear, to resist chemical corrosion, to protect substrates from hostile environments and to resist erosion. For example, ion-assisted vacuum deposition techniques such as cathodic arc plasma deposition (CAPD) have been employed for depositing thin films on substrates to be protected. CAPD processes have achieved superior film bonding and higher densities than more conventional ion plated films.
The conventional CAPD system includes a vacuum chamber, a cathode, an arc power supply, means of igniting an arc on the cathode surface, an anode and a substrate bias power supply. A vacuum arc is employed to evaporate the source material which functions as the cathode in the arc circuit. A voltage in the range of 15 to 50 volts is typically employed to sustain the arc. The voltage level is dependent upon the cathode material.
Arcing is initiated by applying a high voltage pulse to an electrode near the cathode and/or by mechanical ignition. Evaporation occurs due to high velocity arc spots traversing across the cathode surface at velocities as great as 100 m/second. The arc spots carry high current densities and are sustained by the plasma that is generated by the arc. The high current density results in flash evaporation of the source material. The resulting vapor consist of electrons, ions, neutral vapor atoms and microdroplets. The electrons are accelerated toward a cloud of positive ions. Emissions from the cathode spots remain relatively constant over a wide range of arc currents as the cathode spots split into a multiplicity of spots. The average current carried per spot depends on the nature of the cathode material.